In the description hereinafter and in the subsequent claims, the term “optical code” is used to indicate any graphic representation having the function of storing information encoded through suitable combinations of elements of preset, for example square, rectangular or hexagonal, shape, dark coloured (usually black) separated by light coloured elements (spaces, usually white), such as barcodes, stacked codes, that is with several overlapped sequences of bars, and two-dimensional codes in general, colour codes, etc. The term “optical code” also comprises graphic representations detectable not only within the range of visible light, but also in the range of wavelengths comprised between infrared and ultraviolet.
In optical code readers of the scanning type, a light beam, in particular a laser beam, suitably focused by an objective comprising one or more lenses and possibly also one or more aperture stops, is caused to impinge on a deflection system, generally comprised of a rotating or oscillating mirror, to generate one or more scanning lines through the optical code. In other types of optical code readers, the entire optical code is simultaneously illuminated.
The light diffused by the code is collected by proper optics, possibly comprising the same mirror used for scanning and an objective comprising at least one lens, and conveyed onto a photodetector element, which converts the intensity thereof into an electric signal. The pattern of the electric signal over the time during the illumination of the optical code through the scanning line, or the pattern of the electric signal over the space in case of illumination of the entire optical code, indicates the presence or the absence, as well as the mutual size and possibly the colour, of the elements comprising the optical code. Thus, through such electric signal, suitably processed, it is possible to acquire and decode the optical code.
In order for the intensity of the light diffused by the code to effectively represent the constituting elements of the optical code, the luminous spot impinging onto it at each point of the scanning line must be of suitably small size with respect to the size of the elements of the minimum possible size (optical code “module”) of the type of optical code being read. In case of readers illuminating the entire optical code, the illuminated area must in any case have characteristics suitable for the acquisition of the optical code.
It follows that the design of the focusing objective determines to a large extent the range of resolution of the optical codes readable by the optical code reader.
In addition, the resolution of the optical codes being equal, the apparent size at the optical code reader depends on the distance between the reader and the optical code being read.
It follows that the design of the focusing objective determines to a large extent also the range of usable reading distances of the optical code reader, or the depth of field.
Given the variety of the conditions of application of an optical code reader, generally it is not possible to define an absolute ideal conduct of the focusing objective, but only an ideal conduct of the focusing objective with respect to the conditions of application, that is with respect to the reading of high, medium or respectively low resolution codes, and with respect to the reading of optical codes at a long, medium or short distance from the optical code reader.
In case of optical code readers provided with static focusing objective, the focusing distance is determined during manufacturing of the reader. The manufacturer thus provides various models of optical code readers having different focusing characteristics. The readers provided with a static focusing objective are, as far as focusing is concerned, extremely accurate and reliable in that they do not have moveable parts, and also individually economical. However, the need to manufacture and store various models increases general costs. Furthermore, from the user's point of view, the optical code readers provided with a static focusing objective are disadvantageous in that they force the user to choose the model in advance and to an integral replacement in case of change of the conditions of application, for example in case the optical code reader requires to be rearranged, for any reason whatsoever, with respect to a conveyor belt of objects marked by optical codes. Choosing the model of each optical code reader in advance is extremely disadvantageous in case of large industrial applications, where there is a plurality of optical code readers.
For these and other reasons, optical code readers provided with a system for adjusting the focusing distance within a given range are known. More in particular, such optical code readers are divided into two classes, those with automatic adjustment and those with manual adjustment.
In both cases, the system for adjusting the focusing distance comprises a mechanism for displacing the focusing objective, or at least one lens thereof, with respect to the light source. With respect to the optical code readers provided with a static focusing distance, the cost and complexity of an optical code reader provided with an adjustable focusing distance are consequently higher, but the production of a complete range of readers is not necessary and, particularly advantageous for the user, each optical code reader is easily adaptable to various conditions of application.
In case of optical code readers provided with an automatic adjustment of the focusing distance, the displacement mechanism comprises an electrical actuator, for example a linear electrical motor.
The operation of the electrical motor can be controlled, during normal functioning of the optical code reader, depending on the distance of the code to be read, detected automatically by the optical code reader itself or by a distance measurer associated thereto. Such optical code readers provided with “dynamic” adjustment of the focusing distance are also known as readers with autofocus. An optical code reader provided with autofocus is for example described in EP 0 786 734 A2.
The optical code readers provided with an automatic adjustment of the focusing distance can also be underused, setting the desired focusing distance only in a configuration or installation procedure, by setting an operation parameter of the optical code reader. In case of this adjustment of the “static” type, during normal operation the optical code reader automatically maintains such desired focusing distance.
The optical code readers provided with an automatic adjustment of the focusing distance are obviously extremely versatile with respect to the application conditions and, in case of autofocus, they allow optimal reading performance even in applications where there is a wide range of variability of the reading conditions.
However, with respect to the optical code readers provided with static focusing distance, they are more complex and substantially more costly, due to the presence of the electric actuator and the related controller. In addition, they have a potential drop of reliability due to the presence of the actuator itself.
In most industrial applications, for example applications for sorting objects in automatic warehouses, applications on production lines and small conveyor belts, etc, the optical code readers are typically mounted on special supports, over object moving means, such as conveyor belts.
In most of such applications, the reading distance range is limited by the variability of the size of the objects, and the depth of field of an optical code reader is generally enough to cover such range. In these cases the optical code readers provided with automatic adjustment of the focusing distance, in particular with autofocus, are overdesigned and at the same time potentially less strong and reliable.
In such applications, the optical code readers provided with manual adjustment of the focusing distance represent the best compromise in terms of versatility, costs, reliability and construction simplicity, allowing for variable reading distances, but being free of electric actuators.
In case of optical code readers provided with manual adjustment of the focusing distance, the displacement of the focusing objective to change the focusing distance occurs through a manually operated element, such as a screw coupled to a female screw.
Manual adjustment of the focusing distance generally occurs during the step of installation of the optical code reader, in such a manner to adapt it to the characteristics of the application. Such installation procedure requires the intervention of a sufficiently skilled operator, capable of estimating the suitability of the focusing upon a preliminary analysis of the application and of the characteristics of the optical code reader, in particular the reading diagrams depending on the reading distance and the resolution of the code.
Manual adjustment of the focusing distance during the step of installation of the optical code reader can be facilitated by indicating the current focus position to the installer, for example through a mechanical system such as a graduated scale in the proximity of the adjustment screw or an electronic system such as a position sensor correlated to the focusing objective or to a moveable part of the focusing adjustment system, whose output is suitably processed by the electronic system of the reader and displayed on a display device such as a display.
In order to guarantee maintenance of the reading performance of the optical code reader, the adjustment performed during the step of installation must however remain unchanged during normal operation of the reader after installation, the application conditions being equal.
To this end, the manual operated element, generally, is not directly accessible at the outside of the housing of the reader, rather it is in a recessed position inside the same, typically accessible only upon removal of a suitable protection element such as a plug or cover. Said protection element has the double function of concealing the adjustment system itself in order to avoid inadvertent actuation, and preventing it from being tampered with, as well as ensuring sealing of the reader with respect to the external elements such as water and dust. Thus, by its nature, the protection element is a hindrance during the required and intentional manual adjustment, in that it entails the need of removing it and the risk of improperly remounting it thus jeopardising its sealing function.
However, regardless of inadvertent actuation of the manual element, manual adjustment of the focusing distance set during the installation of the optical code reader can be lost due to vibrations, for example transmitted by a conveyor belt on which the reader supporting structure is fixed, or due to impacts caused by people or objects, such as the packages moving on the conveyor belt.
In case of optical code readers provided with manual adjustment of the focusing distance of the prior art, loss of the adjustment carried out during the step of installation can seriously jeopardise the reading performance of the optical code reader. In case of applications in which there is an optical code reader operator, the latter must undertake to estimate the degree of the drop of performance and understand that the drop of performance is due to loss of the adjustment of the focusing distance, then request the intervention of a more skilled operator who shall undertake restoration of the adjustment. Furthermore, the problem of loss of the adjustment of the focusing distance is particularly serious in case of applications of the “unattended scanner” type, where there is no constant supervision by the personnel in charge of the optical code reader, in that the drop of performance can lead to a costly stop of the conveyor belt or of the production line.
The subsequent manual focusing adjustment is, in case of the optical code readers of the prior art, still quite a slow and demanding procedure, requiring as mentioned beforehand the intervention of a skilled operator.
Thus, the technical problem at the basis of the present invention is to overcome the outlined drawbacks of the prior art, providing an optical code reader provided with a variable focusing distance and hence versatile with respect to the application conditions, being both of a low cost and high reliability and easy to manufacture, but that simplifies the adjustment of the focusing distance and in particular allows an easy and quick restoration of the focusing distance adjustment in case of loss thereof.